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images [2016/02/22 17:18]
christian [Disclaimer] 		images [2016/09/24 08:15] (current)
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 ====== Images ====== ====== Images ======
  
-Bitmapped images are described in chapter 8.9 //Images// on page 203 of the {{pdf32000_2008.pdf#page=211|PDF specification}} on quite readable 14 pages.+Bitmapped images are described in chapter 8.9 //Images// on page 203 of the {{pdf:pdf32000_2008.pdf_page_211|PDF specification}} on quite readable 14 pages. 
 + 
 +The implementation is in package ''PDF Images''.
 ===== Usage ===== ===== Usage =====
  
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     renderer paintXObject: anImage asPDF]].     renderer paintXObject: anImage asPDF]].
 </code> </code>
 +
 +{{pdf:demo20_imagesusage.pdf}} shows the result with an example image.
 +
 +{{pdf:demo21_images.pdf}} shows some PDF features of images (masking, rotation, interpolation, inverting and alpha blended images).
  
 ===== Object Models ===== ===== Object Models =====
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 </code> </code>
  
-In PDF images are defined by the number of bits per color component (1, 2, 4, 8 or 16 bit). The colorspace defines the number of components and their layout for a pixel. Any colorspace permitted in PDF can be used (see [[Colors]]). For images converted from Smalltalk, only ''/DeviceRGB'' (3 components), ''/DeviceGray'' (1 component) and ''/Indexed'' (1 component) are relevant. The ''/Indexed'' colorspace in PDF can only hold up to 255 colors.+In PDF images are defined by the number of bits per color component (1, 2, 4, 8 or 16 bit). The colorspace defines the number of components and their layout for a pixel. Any colorspace permitted in PDF can be used (see [[:colors]]). For images converted from Smalltalk, only ''/DeviceRGB'' (3 components), ''/DeviceGray'' (1 component) and ''/Indexed'' (1 component) are relevant. The ''/Indexed'' colorspace in PDF can only hold up to 255 colors.
  
 A ''PDF.ImageXObject'' is a stream whose contents is a byte string with the pixel bits. A row ends at the byte boundary. A row with one pixel with 3 components of 1 bits (= 3 bits), uses 1 byte (8 bit) for the row. As any stream, ImageXObjects can be compressed using filters. By default, ''/FlateDecode'' (zip) is used. A ''PDF.ImageXObject'' is a stream whose contents is a byte string with the pixel bits. A row ends at the byte boundary. A row with one pixel with 3 components of 1 bits (= 3 bits), uses 1 byte (8 bit) for the row. As any stream, ImageXObjects can be compressed using filters. By default, ''/FlateDecode'' (zip) is used.
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 The conversion methods are implemented in the ''Graphics.Image'' hierarchy. To convert a Smalltalk image to PDF with ''asPDF'', the method ''writePixelsTo: anImageXObject'' transfers the actual pixels from the Smalltalk to the PDF image. The other direction uses the method ''readPixelsFrom: anImageXObject'' which transfers the PDF pixels to the Smalltalk image. The conversion methods are implemented in the ''Graphics.Image'' hierarchy. To convert a Smalltalk image to PDF with ''asPDF'', the method ''writePixelsTo: anImageXObject'' transfers the actual pixels from the Smalltalk to the PDF image. The other direction uses the method ''readPixelsFrom: anImageXObject'' which transfers the PDF pixels to the Smalltalk image.
 +
 +{{ :pdf:bitfiddling200x266.png?nolink|Work sketch for bit fiddling}}
  
 The default behavior is to transfer the pixels one by one. For each pixel, the bits are read from the specified location in the source image bytes and interpreted as color (''valueAtPoint:''). This color is then converted to the target bits which are written to the specified location in the target image bytes (''valueAtPoint:put:''). While these two pixel accessors are correct and well tested for any kind of image, they are very slow. The default behavior is to transfer the pixels one by one. For each pixel, the bits are read from the specified location in the source image bytes and interpreted as color (''valueAtPoint:''). This color is then converted to the target bits which are written to the specified location in the target image bytes (''valueAtPoint:put:''). While these two pixel accessors are correct and well tested for any kind of image, they are very slow.
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 ===== To be done ===== ===== To be done =====
  
-Although all Smalltalk images can be used for PDF, not all PDF images can be transformed to Smalltalk images. For one, several {{pdf32000_2008.pdf#page=22|filters}} specific to images are not implemented:+==== Filter ==== 
 + 
 +Although all Smalltalk images can be used for PDF, not all PDF images can be transformed to Smalltalk images. For one, several {{pdf:pdf32000_2008.pdf_page_22|filters}} specific to images are not implemented:
   * **RunLengthDecode** 8 bit monochrome images   * **RunLengthDecode** 8 bit monochrome images
   * **CCITTFaxDecode** CCITT encoded 1 bit monochrome images   * **CCITTFaxDecode** CCITT encoded 1 bit monochrome images
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   * **JPXDecode** JPEG2000 encoded grayscale or color images.   * **JPXDecode** JPEG2000 encoded grayscale or color images.
  
-This means that it is not possible to extract such images from PDF. Nor is it possible to store images in the most efficient way in a PDF. This feature is valuable and I hope to implement it in the not too distant future.+This means that it is not possible to extract such images from PDF. Nor is it possible to store images in the most efficient way in a PDF. This feature is valuable and I hope to implement some of the filters in the not too distant future.
  
 Secondly, PDF can have images in other colorspaces than RGB or Grayscale; most notable is ''/DeviceCMYK'' for print. For correctly extracting such images, proper color conversions to RGB need to be implemented. This feature is not intersting to me at the moment. Secondly, PDF can have images in other colorspaces than RGB or Grayscale; most notable is ''/DeviceCMYK'' for print. For correctly extracting such images, proper color conversions to RGB need to be implemented. This feature is not intersting to me at the moment.
 +
 +==== Inlined Images ====
 +
 +Images in PDF can be inlined in the /Contents stream instead of storing them in the /Resources as /XObject. Only a subset of legal PDF images can be inlined and it is discouraged for large images. Even though, I have not seen such image in a real-world PDF, this feature should be implemented for completeness.
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